Fluidized beds of sand, activated carbon, or other media have been used for a wide variety of aerobic and anaerobic waste treatment processes. Because of their extremely high surface areas for biological growth and high hydraulic loading rates, fluidized beds have demonstrated significant cost and performance advantages for biological treatment of industrial, process, and municipal waste water. Nevertheless, a significant number of these systems which were built over the last twenty years have become inoperable or have been subject to excessive maintenance. These operational problems can be traced to the chronic clogging of the inlet distribution system which distributes the influent at the bottom of the filter bed.
The inlet distribution system of these biological fluidized beds can become clogged for two principal reasons. First, cessation of normal flow through the filter causes intermittent fluid backflow. This condition draws the media bed into the inlet distribution system and causes clogging. The problem occurs because the media has a greater density than the fluid and therefore tends to displace the fluid in, for example, the lateral elements of the distribution plumbing. Conventional check valves which attempt to prevent this require some degree of backflow in order to activate them. They are thus ineffective in preventing clogging of the inlet system. Moreover, conventional check valves are subject to sticking when even small amounts of media become caught in them. The second, principal cause of clogging occurs when foreign materials become lodged in the laterals or openings in the distribution system and impedes normal operation.
A third factor may play a role in clogging the laterals. The flow in the laterals is turbulent. Pressure fluctuations associated with such flow conditions appear to draw media into the laterals even under normal operating conditions.
Since it is critical to maintain an even flow distribution through the system so that the bed is uniformly fluidized, even a few nozzles becoming inoperative permits the media above them to settle into a heavy dense unfluidized mass. The remaining nozzles receive more flow and, as a result, the media above them becomes even more fluidized and tends to be deposited over the dead zone, increasing its size and making the problem worse. This phenomenon of excessive fluidization and subsequent media deposition typically worsens until a major part of the bed is no longer fluidized.
Since the distribution plumbing is typically under one to several hundreds of tons of media, clearing media from the reactor to unclog even a few nozzles is a maintenance nightmare, being difficult, messy, time-consuming and expensive.